Electrical apparatus for differential current protection

ABSTRACT

An electrical apparatus for magnetothermal and differential protection, of the type comprising a device for sensing differential currents associated with a magnetothermal protection switch, comprises a differential current sensor connected to an input of a controlled gain signal amplifier, in turn connected to a plurality of light indicators arranged to go on progressively in dependence on the sensor. The apparatus provides a quantitative and qualitative measurement of the stray current.

This is a continuation of application Ser. No. 07/349,631, filed Jun. 8,1989, now abandoned, which is a divisional of application Ser. No.200,690, filed May 31, 1988, now U.S. Pat. No. 4,851,950.

This invention relates to an electrical apparatus for magnetothermal anddifferential protection, of the type comprising a differential currentsensing device associated with a magnetothermal protection switch.

Electrical protection apparatus of the type noted above are generallydesigned for use with industrial three-phase low tension power systems.

Known in that field of application are wall-mounted apparatus equippedwith gauging instruments which are connected serially in the electricpower system and operative to monitor, ahead of the installation of aprotection apparatus, the appearance of differential currents, commonlyreferred to as stray currents.

Such gauging instruments, white being satisfactory performance-wise andserving a number of electrical failure sensing functions, are merelyprovided with an emergency light indicator which is turned on upon apredetermined threshold value being exceeded of an electric quantity tobe gauged.

Such instruments cannot, therefore, provide a quantitative measurementof a stray current and prevent untimely operation of tie protectionapparatus.

The prior art further proposes the use of portable gauging instruments,commonly referred to as testers, whereby various checking operations canbe carried out in connection with power system safety regulations, whitealso measuring stray currents.

However, gauging instruments of the latter type have the disadvantagethat they cannot be combined with the protection apparatus and can onlypermit of an occasional check on the state of an electric power system.

Furthermore, the electronic sensing devices of the aforesaid protectionapparatus, are connected "downstream" of the protection switch via asolenoid which acts on the trip-off linkage of the switch.

This solenoid, additionally to opening the switch, affords protectionfor the electronic sensing device against overvoltages in the powersystem or in the event of the device itself being shorted, but has thedrawback of being ineffective on the occurrence of an interruption inthe electric supply phase.

The technical problem which underties this invention is to provide anelectrical apparatus for magnetothermal and differential protectionwhich has such structural and operational characteristics as to affordsensing and a quantitative measurement of differential currents in anelectric power system, by overcoming the drawbacks mentioned above inconnection with similar prior apparatus.

This problem is solved by an apparatus as indicated being characterizedin that said device comprises:

a sensor for sensing said differential currents;

a controlled gain signal amplifier connected to the output of saidsensor; and

a plurality of light indicators connected to the output of saidamplifier and adapted to be turned on progressively in dependence onsaid sensor.

The features and advantages of an apparatus according to the inventionwill be more clearly understood from the following detailed descriptionof an exemplary embodiment thereof, to be read in conjunction with theaccompanying illustrative drawings.

In the drawings:

FIG. 1 is a block diagram showing schematically the protection apparatusof this invention;

FIG. 2 is a wiring diagram of a detail of the apparatus shown in FIG. 1;and

FIG. 3 shows a wiring diagram of another detail of the apparatus shownin FIG. 1.

With reference to the drawing views, the numeral 1 generally designatesa schematically depicted electrical apparatus for protection fromdifferential currents Id, also called stray currents, appearing on athree-phase low tension power system. The power system includesconventional phase conductors R, S and T, and a neutral conductor N,which form a four-pole line 44 for a three-phase voltage system designedto power electric loads, not shown because known per se, within a rangeof values from 50 Volts to 500 Volts AC.

The apparatus 1, which is connected serially in the four-pole powersystem, comprises an automatic switch 3 for magnetothermal anddifferential protection, connected serially in the line 44 and having adevice 2 associated therewith for sensing said differential currents Id.

The apparatus 1 comprises a plurality of electronic components, to bedescribed in detail hereinafter, which are powered from a supply circuit4 whose input end is connected to said conductors R, S and T of thethree-phase voltage system.

The supply circuit 4 outputs a pair of voltage poles, respectivelypositive and negative ones, indicated at Vc and Ve and being at levelsequal to 10 Volts and -7.5 Volts. The supply circuit 4 is designed toremain operative even with an outage of any of the three phase voltages.More specifically, the supply circuit 4, shown more clearly in FIG. 2,has at its input a three-phase bridge circuit 10 composed of six diodeswhich form a pair of diode bridge circuits 11 and 12 sharing a pair ofsuch diodes.

Referring again to FIG. 1, the supply circuit 4 has its input endconnected to the conductors R, S and T "downstream" of the switch 3 viaa solenoid 5 adapted to operate the break linkage of the switch 3 bymeans of its plunger 6.

The solenoid 5 comprises first 7 and second 8 windings which arestructurally independent of each other and respectively connectedbetween the phase conductor R and an input 13 of the first bridgecircuit 11 (see FIG. 2), and between the phase conductor S and an input14 of the second bridge circuit 12. The second input 15 of either diodebridge circuits 11 and 12 is connected directly to the third phaseconductor T.

Referring to FIG. 1, the sensing device 2 comprises a toric transformer9 having high magnetic characteristics which is arranged to encircle thephase conductors R, S and T and the neutral conductor N to sense currentunbalance occurring therebetween on the appearance of ground-going straycurrents Id. Referring to FIG. 3, the transformer 9 is provided with awinding 16 having one end connected to the positive pole Vc via aresistor R1 and the other end connected, via resistors R2 and R3, to acontact 20 of a test key 21 on the apparatus 1 which is pivoted to aterminal 23 connected to the negative pole Ve and movable from a firstposition of contact with the center terminal 18 of a selector 24 to asecond position associated with the contact 20.

An operational amplifier Al has its output fed back to the non-invertinginput via said resistor R2, with the non-inverting input being furtherconnected to the pole Vc via a resistor R4 and to said contact 20 viathe resistor R3. The output from the amplifier Al is also fed back tothe inverting input via a resistor R5; that inverting input is furtherconnected to the terminal 23 through a parallel-connected resistor R6and capacitor C1. The transformer 9 further includes a secondary winding17, having approximately one thousand turns, which forms a sensor ofsaid differential currents Id and has its ends connected to a signalamplifier 30.

Further, one end of the secondary winding 17 is connected to theterminal 23 of the test switch 21 via a first resistor RS1 in a group offour resistors of a resistive divider making up the selector 24.

The amplifier 30 is of the controlled gain type tied to the dividerselector 24, wherein the remaining resistors RS2, RS3 and RS4 have eachone end connected to the secondary winding 17 and the other endavailable to receive the moving contact 19 associated with the centerterminal 18 of the selector 24.

The output of the amplifier 30 is connected to both the input of adifferential current threshold sensor 32 and the input of an outputsignal peak sensor 31.

The output of the sensor 32 is connected to the input of a trip-offactuator 33 operative to drive the switch 3 to break on a differentialcurrent being sensed whose value exceeds a predetermined threshold.

The output of the peak sensor 31 is instead connected, via a voltagedivider 34 to the input of a driver circuit 35 for light indicators ofthe LED type, denoted by the reference numerals 36 to 40.

More specifically, the aforesaid amplifier 30 comprises an operationalamplifier A2 having its inverting input connected via a resistor R7 toone end of the secondary winding 17 of the transformer 9 and thenon-inverting input connected to the other end of said winding 17 aswell as to ground.

The output from the amplifier A2 is fed back to the inverting input viaparallel connected resistor R8 and pair of push-pull connected Zenerdiodes Z1 and Z2. The output of said amplifier A2 is further connectedto the input of a bridge 41 firmed of resistors R9, R10, R11 and R12 andconnected with its output to a second operational amplifier A3.

Connected in parallel with the resistor of the bridge 41 are a pair ofdiodes D1 and D2 having the output connected therebetween of a thirdoperational amplifier A4 which has the inverting input connected betweenthe resistors R9 and R11 of the bridge 41, and the non-inverting inputgrounded.

Also grounded is the non-inverting input of the operational amplifierA3, whose output is connected directly to the base B1 of a pnp-typetransistor T1 having the emitter E1 connected to the base B1 via a diodeD3 and to the inverting input of that same amplifier A3.

As shown in FIG. 2, the collector C1 of the transistor T1 forms theoutput of the amplifier 30 and is connected directly to the invertinginput of an operational amplifier A5 incorporated to the thresholdsensor 32. The amplifier A5 is connected electrically between thepositive pole Vc and the negative pole Ve, and has its inverting inputconnected, in turn, to the negative pole Ve through parallel connectedcapacitor C6 and resistor RS. The non-inverting input of the amplifierA5 is both connected to ground, via a resistor R13, and to the negativepole Ve, via a resistor R14.

The output of the amplifier A5 constitutes the output of the thresholdsensor 32 and is connected, via a variable resistor RT, to the invertinginput of an operational amplifier A6 incorporated to the trip-offactuator 33. That amplifier A6 has its inverting input further connectedto both the positive pole Vc through parallel connected capacitor C8 andresistor R15, and the negative pole Ve via a resistor R16. Further, theoutput of the amplifier A6 is fed back to the non-inverting input via aresistor R17; that input is also grounded via a resistor R18.

The amplifier output is also connected to the pole Vc via a resistor R19and to the the base 82 of an npn-type transistor T2 having the emitterE2 connected to the pole Ve and the collector C2 connected to the poleVc via a diode D5.

Between the collector C2 and the pole Vc is an actuator 42 consisting ofa silicon controlled rectifier Scr which is adapted to bypass thethree-phase bridge circuit 10 of the supply circuit 4. In particular,the rectifier 42 has a gated input G adapted to receive a signal on adifferential current Id being sensed which exceeds the cut-in threshold;upon receiving that signal on the input G, the rectifier 42 bypasses theinputs of the supply circuit 4.

Advantageously, as shown in FIG. 3, the peak sensor 31 comprises a diodeD4 connected to the negative pole Ve via a capacitor C7. Between thediode D4 and the capacitor C7 and an input Il of the driver circuit 35,there is a resistor R20.

An additional resistor R21 connects between the input Il and thenegative pole Ve, which forms, in cooperation with the aforesaidresistor R20, the voltage divider 34.

The driver circuit 35 includes additional inputs 12 and 13 respectivelyconnected to the positive Vc and negative Ve poles, and has a pluralityof outputs, denoted by the reference characters 14 to 18, each connectedto one of the LED-type light indicators 36, 37, 38, 39 and 40. Theselight indicators are cascade connected to one another and the last one,40, is also connected to the negative pole Ve.

The apparatus 1 of this invention is further provided with a selector43, as shown in FIG. 1, consisting of an additional resistive dividerthrough which the trip-off time delay of the switch 3 is adjusted.

The operation of the apparatus according to the invention will be nowdescribed.

The secondary winding 17 of the transformer 9 senses a current unbalancebetween the phase conductors R, S and T and the neutral conductor N inthe presence of a ground-going leak from a differential current Id.

The signal sensed by the secondary winding 17 is amplified through theamplifier 30 and supplied to the input of the threshold sensor 32 which,in the event that the differential current exceeds a predeterminedthreshold level, sends a signal to the input of the trip-off actuator 33to activate the rectifier 42. Upon operation of the rectifier 42, thethree-phase bridge circuit 10 of the supply circuit 4 is short-circuitedand the phase supply voltage is applied directly to the windings 7 and 8of the solenoid 5 to open the switch 3.

The peculiar structure and the connections of the solenoid 5 windingsenable it to become operative even without one of the voltage phases. Asan example, should there occur a phase voltage outage on the conductorR, the winding 8 of the solenoid 5, and accordingly, the supply circuit4 for the device 2 would still be active.

The threshold value of the cut-in current may be changed by means of theselector 24 until differential currents of 30 mA are sensed; withcurrents of such low values, the moving contact 19 of the selector 24 isleft in the open position.

The signal output by the amplifier 30 is further applied to the input ofthe sensor 31, which extracts the peak value of the signal to pass it,via the voltage divider 34, to the input I1 of the driver circuit 35,which will turn on the LED indicators.

The circuit 35 is designed such that the indicators 36 to 40 go onprogressively as the sensed differential current increases, and byincrements thereof of about 15%. Thus, the first LED 36 will go on forvalues of Id equal to 15% of the threshold value set by means of theselector 24; the second LED 37 will go on for differential currentvalues equal to 30% of the threshold value, and so on until the lightingof all five LEDs indicates that a current value has been reached whichcorresponds to 75% of the cut-in threshold.

CEI (Comitato Elettrotecnico Italiano) Standards provide fordifferential protection to cut in on detection of a stray current withinthe range of 50% to 100% of the set threshold level.

Accordingly, lighting of the first three LEDs 36, 37 and 38 will resultfrom a lower differential current being sensed than the cut-in thresholdlevel established by CEI Standards for differential protection.

Lighting of the fourth LED 39 is a warning of possible intervention ofthe protection, and of the fifth LED 40 of that the apparatus 1 hasreached the limit of the protection cut-in band.

By manipulating the selector 24 from the outside, the threshold valuefor the cut-in current can be changed, automatically adapting theresponsiveness of the apparatus 1 and of the driver circuit 35 for thelight indicators.

The apparatus of this invention has a major advantage in that itprovides a quantitative measurement of the differential currents andcombines the protection, control, and warning functions in a consistentmanner.

By having the light indicators color coded, the task of the installer isfacilitated because he can select the most appropriate level for thedifferential protection cut-in threshold to suit the leakagecharacteristics of a power system for which this apparatus is beinginstalled.

Further, the inventive apparatus affords the important advantage offully retaining its effectiveness even on the occurrence of an outage onany of the supply voltage phases, and that by locating the trip-offsolenoid upstream of the differential sensing device, that electronicdevice can be effectively protected in the event of an overvoltage onthe power supply line.

I claim:
 1. An electrical apparatus for differential current protectionassociated with a protection switch, comprising:a sensor for sensingdifferential currents in a three phase system; a controlled gain signalamplifier having an input connected to said sensor and receiving at itsinput a signal corresponding to said differential currents and having anoutput when said sensed differential currents have exceeded apredetermined threshold current; a plurality of indicators connected toan output of said amplifier and adapted to be activated progressively inresponse to said differential current for providing a quantitativeindication of said sensed differential currents as a percentage of saidpredetermined threshold current; and a trip off actuator having an inputconnected to said output of said amplifier, and having first and secondwindings respectively connected to first and second phase voltages insaid three phase system, so that when said sensed differential currentshave exceeded said predetermined threshold current, said actuator openssaid switch in response thereto.
 2. An apparatus according to claim 1,wherein said sensor comprises the secondary winding of a transformer. 3.An apparatus according to claim 1, wherein said light indicators areconnected to a driver circuit and wherein said driver circuit isconnected to said amplifier via a peak sensor and voltage divider.
 4. Anelectrical apparatus for differential current protection, of the typecomprising a differential current sensing device associated with aprotection switch, characterized in that said sensing device comprises:asensor for sensing differential currents in a three phase system,wherein said sensor comprises the secondary winding of a transformer; acontrolled gain signal amplifier connected to said sensor to generateamplified output signals related to said sensed differential currents; athreshold selector tied operatively to said controlled gain signalamplifier such that a threshold may be selected by controlling theamplifier gain; a plurality of light indicators connected to saidamplifier and adapted to be turned on progressively in response to saidamplified output signals; a threshold sensor for sensing when saidamplifier output signals related to said sensed differential currentshave exceeded a selected threshold; and a trip-off actuator to open saidprotection switch in response to said threshold sensor sensing anamplified output signal exceeding said selected threshold wherein saidactuator comprises first and second windings respectively connected tofirst and second phase voltages in said three phase system.